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"he locks the instance"
An insert does not 'lock' an instance (unless you consider an uncommitted transaction a lock that prevents normal instance shutdown).
It gets a share lock on the table being inserted into.
The inserted row is 'locked' but if uncommitted then no other session would be able to see it from an UPDATE, DELETE, INSERT...SELECT or SELECT FOR UPDATE.
SELECTs don't take locks anyway, so they would have no problems.

INSERTs can cause concurrency issues for other INSERTs/UPDATEs/DELETEs if there is a bitmap index. (Effectively a single row in a bitmap index covers multiple table rows, so changing one table row can 'lock' other table rows).